1) Fatality rate is based on hospital care being available for people who do get sick. This will include some nonzero percentage of otherwise healthy young people. Some of these patients will need advanced support, including ventilators.
2) Assuming you could contain to 40M young people (likely impossible), if even .5% of them need hospitalization, the UK hospital system will be overrun. (Not even accounting for all the other people who need hospitalization.)
3) Therefore: the low fatality rate assumption in #1 would likely not hold.
TL;DR; the absolute numbers are too big relative to health system capacity.